1. Field of the Invention
The present invention relates to apparatus for detecting a fuel mixture ratio and in particular to an apparatus which detects the fuel mixture ratio of mixed fuel containing two different kinds of fuel, for example like gasoline and alcohol. More precisely it relates to apparatus for detecting a fuel mixture ratio which detects optically, by utilizing a change of an absolute refractive index in accordance with the fuel mixture ratio.
2. Prior Art
Two kinds of apparatus for detecting a fuel mixture ratio have been proposed and developed. One kind detects the mixture ratio from a dielectric constant of mixed fuel as mentioned in Published Unexamined Japanese patent application No. Sho 56-98540, and another kind detects the mixture ratio from the absolute refractive index of mixed fuel as mentioned in U.S. Pat. No. 4,438,749.
Thus, the former of the above-mentioned detection apparatus detects the fuel mixture ratio utilizing the fact that the dielectric constant of the mixed fuel changes in proportion to the fuel mixture ratio, according to the difference of the dielectric constant of the fuel to be mixed, while the latter detection apparatus detects the fuel mixture ratio optically, utilizing the fact that the absolute refractive index of the mixed fuel changes in proportion to the fuel mixture ratio, according to the difference of the absolute refractive index of the fuel to be mixed. In the case of detecting the dielectric constant of fuel like the former apparatus, there is a problem that the detecting result varies widely, according to the temperature or the flow of fuel, or to impurities. Therefore, the other kind which detects the change in the absolute refractive index of the fuel optically, like the latter one above-mentioned, is favored.
In the prior apparatus for detecting the fuel mixture ratio by using the change in the absolute refractive index of the above-mentioned mixed fuel, a glass stick with a light emitting element and a light receiving element on opposite ends is mounted in the mixed fuel, the light emitting element emits light toward the side of the glass stick (the boundary surface with fuel) and the light receiving element receives light that is reflected from the boundary surface, as mentioned in U.S. Pat. No. 4,438,749.
Since a critical angle wherein illuminated light from the light emitting element all reflects varies in accordance with a change of a reflective index of mixed-fuel and reflected light to be received at the light receiving element varies according to fuel mixture ratio, this apparatus aims to detect the change by a detecting signal from the light receiving element and to detect the fuel mixture ratio. However, in this case, the light emitting element is attached to one end of the long glass stick, and therefore, there exists a problem that it is difficult to determine an illuminating angle correctly and that detecting signals of each detection apparatus scatter. This means that if an illuminating angle from a light emitting element scatters, the quantity of light reflected from the boundary surface between the side of a glass stick and fuel changes and an output signal from a light receiving element, in other words a detecting signal of fuel mixture ratio scatters.
In the above-mentioned detection apparatus, the light emitting element and a light receiving element are attached to opposite ends of the glass stick, face to face and the light receiving element receives direct light from the light emitting element with reflected light from the boundary surface which varies according to fuel mixture ratio. Therefore, there is a problem that the change of reflected light that indicates fuel mixture ratio cannot be captured sufficiently as detecting signals and that it is weak because of included noise signals.